Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/frames/default/frame_25.txt

@@ -1,17 +0,0 @@
-                                      
-             _+=*;++++_               
-           ,!*,*`_;\|*||,             
-          /|//,,".|+\=\|||_           
-         |+*| /! =| "\|,*\/           
-        ,__\,/'^;/_|" |//\/*          
-        \,~|~_*+.^|: /|,|//|,         
-        |/|*| |__/\_/|\/_"|=|         
-        |||/|."!~_=\/|\_~=||\         
-       ^+\|"|__====+~|\\|+*|\         
-        /-"|/|,|_||;*_|\*=/\|         
-        \~*/\|`"""""'+/|\|/|`         
-         |_"|;+;\-,*:_/,//|/          
-          ^`^|_\_*;/,^/_||/           
-           \.;\\_*=|**!*|*            
-             ~,"*\+,_+|*              
-